John Hazel wrote a fairly sophisticated Excel program back in 1994 that
calcs sailplane lauch profiles and resultant altitude gains with different
launch styles. His program takes into account DC motor torque profiles,
changes in drum diameter as line winds up, aerodynamic drag of towline
(given its variable speed with its distance from the plane/turnaround, wind
gradient, etc.), wind gradient with altitude, glider max CL, CD and wing
area, line stretch, distance from launch point to turn-around, etc., etc.
He wrote an article in RCSD sometime around 1994 (??) describing launch
techniques as analyzed by his software and confirmed by numerous test
flights with an altimeter watch in both our gliders (a 2 meter DUCK and 3.5
meter MAGIC). In addition he verified some typical winch current draws by
measuring the amp draw during dozens of competition launches. There IS a
specific launch and flight profile to fly in order to get max altitude for a
launch, and this varies with the type of glider and somewhat with the
headwind.
I designed the "HERETIC" 2 meter glider in 1994 to specifically get maximum
altitude from launches (with little consideration of soaring ability). It's
launch and zoom were absolutely stupendous! Almost scary! When the design
was modified to improve handling/soarability the launch suffered some but
still is good.
Line stretch of monofilament towline is an essential part of launch
performance with F3B and F3J sailplanes (i'm relatively inexperienced here).
Line length is measured with nominally "no load". It stretches MANY feet,
depending on diameter and load. Other F3B fliers know MUCH more on this I
suspect.
Dan deVries
DEVRIES DESIGN, Inc.
Product Design --+-- Patent/Litigation Graphics
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[EMAIL PROTECTED]
>Message text written by INTERNET:[EMAIL PROTECTED]
>>Contemporary designs with higher loadings, stronger wings and less area
>and
>LESS HIGH SPEED DRAG can take advantage of high line tension. The stretch
>of the line stores energy that can be instantaneously transferred to the
>glider, which accellerates (with its low drag) and then pulls up into a
>vertical climb trading the speed for altitude. No line stretch -- no zoom
>gain.<
>-----------------------
>That brings up a very important point. Line stretch is an important factor
>in optimal energy insertion for glider, in particular to ratchet winch
>system. Prelaunch energy storage, semi-highstart effect for initial
>acceleration and energy insertion. While climbing, pulsing motor current
>result in smooter and efficieny energy build-up, rather than insertion loss
>into heat. Final zoom with stretch line promote constant line tension to
>keep energy insertion smooth and therefore minimum insertion loss. Stretch
>line is practically a linearly gear shift system within a limited dynamic
>range. Any one see the down side for an optimal stretching line?
>
>The next question of "Is it leagle" to use stretching line?
>
>Followed by, "If so, how to determine the tensile constant" for optimal
>energy insertion?
>
>-YK Chan
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